This invention generally relates to electronic systems and in particular it relates to charging the photodiode element in active pixel arrays.
A prior art active pixel light sensing element is shown in FIG. 1. This state of the art active pixel element consists of three NMOS transistors 20, 22, and 24, and a photodiode 26. One of these transistors is called a reset transistor 20 and its function is to charge up the photodiode 26 (nwell) to a preset value at the beginning of each exposure cycle. The reset operation is needed in order to eliminate memory effect from prior exposure.
Biasing conditions used in state of the art 0.18 um technology are as follows: Nwell photodiode 26 is charged up to 2 V during reset. In order to pass 2 V through the NMOS transistor 20, a voltage of 3.3 V (+/xe2x88x9210%) is applied on the gate. When the reset transistor 20 is turned off (gate goes to 0 V), nwell potential at node 32 is pushed down by approximately 100 mV due to charge injection.
Referring to FIG. 1 showing the conventional pixel architecture, the dynamic range at the column output node 30 is calculated as follows:
(1) nwell 26 is charged to 2000 mV when transistor 20 is turned on;
(2) nwell 26 goes to 1900 mV due to charge injection when reset transistor 20 is turned off;
(3) voltage at the source 34 of sense transistor 22 (sense device) is 800 mV=1900 mVxe2x88x92nmos Vt {1100 mV with back bias} (nmos Vt is the threshold voltage of transistor 22);
(4) minimum voltage drop allocated for the current source transistor 28 is 200 mV;
(5) dynamic range at column output node 30 is 600 mV=800 mVxe2x88x92200 mV;
(6) there is no significant voltage drop across column select transistor 24 (column select device).
One of the limitations of this prior art approach (combined with high voltage sense transistor 22) is that dynamic range is limited to 500 mV if the gate voltage on the reset transistor 20 drops to 3.0 V.
A forward biased diode is used to charge up a photodiode rather than an NMOS transistor. This photodiode charging mechanism increases the dynamic range and optical response of active pixel arrays, and improves the scalability of the pixel element.